[dataengines/hotplug] Port code to new connect syntax

Summary:
It wasn't ported before due to the slot being overloaded.

Code became cleaner if we renamed one of the private slots.

Test Plan: Compiles

Reviewers: Plasma, sitter

Reviewed By: sitter

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22841